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Technical
Apart from environmental considerations, the prime reason for the continued growth of powder coatings compared to all other coating technologies, is the ingenuity of the formulators of the resins, curing agents, and additives, who continually expand the qualities of powder coatings to meet new challenges to film performance. There is an on going need for formulators of powder coatings to examine new technologies which may be beneficial to future developments. It is for this reason that I have given comprehensive coverage to the programme for the 9th European Coatings Conference which takes place in Berlin on 6 & 7 June 2002. The theme for this year is “Smart Coatings” and the fourteen papers will provide much thought provoking material for the inventive minds of powder coatings formulators. We have come a long way since the early days of powder coating formulation, and it seems that there is plenty of mileage ahead. - YSid Harrts
Continuous powder coating spray process The standard method for manufacture of thermosetting powder coatings has dominated the market for almost forty years and a number of alternative methods have been developed in recent years although these alternatives have yet to become commercially acceptable. The main drawbacks to the traditional manufacturing technique are: high risk of cross contamination; broad particle size distributions; it is not suitable for low melting point coating systems; and is also unsuitable for processing highly reactive systems. Some years ago, the novel
VAMP process was developed by researchers at Ferro. This process replaced the extrusion, cooling and milling stages of manufacture with a stirred autoclave and a spray tower. In the autoclave a mixture of powder coating components was made with super critical carbon dioxide and, after processing, was sprayed to give a uniformly powdered material. Unfortunately, the costs of this process were prohibitive, due to the large high pressure vessel required for homogenization, the relatively long time in the autoclave, and the discontinuous mode of operation. Another continuous process for manufacture has now been devised based on the PGSS (Particles from Gas Saturated Solutions) technology. In this CPCSP (Continuous Powder Coating Spray Process), the main components of the powder coating mixture, the binder and the hardener are melted in separate vessels to avoid premature reaction of the polymers. Using high pressure pumps, the melts are dosed to a static mixer, and additionally carbon dioxide is dosed to the static mixer with a membrane pump. In the mixer the streams are homogenized and under pressures from 60 to 250 bar the gas is partially dissolved in the melts. This partial solution is then instantly released into a spray tower through a nozzle. The rapid expansion of the gas creates a Joule-Thomson effect which freezes the melt into fine droplets. The new process has been tested with single powder coating components and with complete formulations in polyester, epoxy and acrylic systems. In these experiments, the effect on particle size and distribution by varying the temperature, pressure and type of nozzle was examined. It was demonstrated that tailormade spherical particles with particle size lower than 40
microns and a very narrow particle size distribution could be obtained. Magnified photographs are shown in this paper to illustrate the sphericity of two polyester grades and a rough particle shape for an acrylic powder coating that is claimed to be caused by low process temperatures, resulting in high melt viscosities during the spray process. Also included are graphical representations of the effect of increasing pressure on particle size and the particle size distribution, and the phase behaviour of a polyester system in the presence of carbon dioxide. When the pressure is set at 61 bar only 7wt.% of carbon dioxide is dissolved in the melt and the powder formed with this setting is very coarse and shows a wide particle size distribution. Increasing the pressure to 177 bar causes nearly 16wt.% of carbon dioxide to dissolve in the melt and smaller particles with narrow particle size distributions are formed. Sensitivity of the process is shown by increasing the pressure to 190 bar when only lwt.% extra of carbon dioxide is dissolved in the melt, the particle size is further reduced and an excellent particle size distribution is obtained. Work on this project has been carried out with a laboratory sized plant capable of producing 15 kg of powder per hour. This has already been scaled up to a 50 kg/hour pilot plant and it is hoped to increase capacity to 350 kg/hour. It is claimed that there is a relatively small gas consumption of 0.5-2.0 kg of carbon dioxide per kg of powder coating. Paper entitled“Continuous PowderCoating SprayProcess - A New Technique to
Manufacture Powder Coatings” by M Petermann, E Weidner and V Rekowski of Ruhr University Bochum, and DuPont Performance Coatings, Germany, presented at PCE 2002 conference in Nuremberg on 15-17 Jan 2002. Bound copies of conference papers are available from the organizers, Vincent2 Verlag, Schiffgraben 43, 30175 Hannover, Germany
APRlL
2002
Reduced reactivity polyester resins for primid systems Over the past ten years, powder coatings based on polyesters crosslinked with a Primid hardener supplied by EMSChemie, have become well established. Their main uses are in outdoor applications for coating architectural facades, window profiles or industrial outlets in automotive primers, light fixtures and lawn and garden equipment. While the exterior durability and flexibility are features of these systems, the main criticisms are the high reactivity and the emission of volatile by-products during the curing cycle. The emission, which is usually water from the condensation reaction, combined with the entrapped air between the powder particles, results in problems with the degassing of the film. If the viscosity of the film is too high these trapped vapours will cause blistering and pinholing in the cured film. Improvements can be made by slowing down the viscosity rise and allowing most of these vapours to escape before the film is cured. The resins used for Primid cure are polyesters with carboxy functional end groups which react with the hydroxyl groups in the Primid to form ester bonds. This reaction does not proceed by the usual addition-elimination esterification reaction but via an oxazolinium-carboxylate ion pair intermediate mechanism which is much faster than normal esterification and cannot be controlled by catalysts. A paper by researchers at DSM Coating Resins describes the work that has been carried out to modify the reaction rate of the polyester component. Theoretically, there are several possible solutions to the degassing problem. A lower acid value in the polyester and a corresponding lower level of crosslinker would release water APRIL
2002
vapour at a lower rate, with a lower viscosity rise during cure. Unfortunately, this has an adverse effect on chemical resistance of the film and the corresponding higher molecular weight of the resin reduces the film flow. Degassing agents such as benzoin improve the release of air during cure but they can cause severe film yellowing. While pigments and additives do not affect the reactivity, the structure of the polyester can have a marked effect. A completely new approach is to modify polyester resins, maintaining composition and molecular weight, by altering the nature of the carboxylic end groups. It has been found that aliphatic end groups react slower than aromatic end groups, and that the end groups are responsible for the reactivity with the Primid crosslinker. Work at DSM has shown that reactivity can be influenced by the amount of aliphatic end groups, although side effects are a drop in glass transition temperature (Tg) and reduced water resistance. The use of adipic acid gives a drop in 7-g and a lower melt viscosity. The limitation to this approach is the storage stability of the finished powder coating. The usual components for powder coating polyester resins are: terephthalic acid, isophthalic acid and neopen~l glycol. DSM Resins has found that incorporating a constant level of aliphatic di-acids into the polyester backbone yields resins with different amounts of aliphatic acid end groups. The desired Tg of the polyester determines the amount of aliphatic acids that can be built into the backbone. It has been found that the reactivity of the resins can be varied independently of the Tg. This enables the properties like durability, powder stability and impact resistance to be retained while degassing qualities can be improved.
Physical and chemical properties of a range of experimental resins are tabulated and compared to the values for a comparable commercial resin. Adipic acid was used as the aliphatic acid component in these resins. Recorded acid values are in the same range as that of the commercial resin. The aliphatic acid expressed as a percentage of all end groups varied between 17.3% and 65.4% compared to 4.0% for the standard. Hydroxyl values are also recorded, together with Tg, melt viscosity, gel time, flow characteristics, and the film thickness limit before degassing is observed. All of these experimental resins showed lower reactivity and better degassing limits than the standard, whilst retaining all other properties. The resins with aliphatic end groups are synthesized in different ways. The different process conditions result in different amounts of adipic acid end groups and different degrees of conversion which is illustrated by the hydroxyl value. The reactivity of the crosslink reaction is measured by FT-IR analysis and by the gel time. Total conversion is independent of the amount of aliphatic end groups, but the conversion rate is related to the amount of aliphatic end groups. After IO minutes at the curing temperature, resins with a higher amount of aliphatic end groups have reached a lower end conversion since they have not completed their curing reaction, apart from the resin with the lowest amount of aliphatic end groups which has already reached its highest level of conversion, possibly due to the fast rate of reaction that restricts the mobility of the end groups and prevents further reaction. Gel time is also a measure of reactivity and this property depends on the amount of aliphatic end groups and the method of synthesis. Film flow
3
depends on rate of cure, but other factors can be viscosity, free acid content or particle size. The resins with aliphatic end groups flow as well as or better than the commercial standard. Degassing is found to improve with gel time and a lower reactivity increases the layer thickness at which blisters will occur. Impact resistance can only be achieved when a minimum amount of conversion is attained. In the tests with the trial resins, increasing the aliphatic end groups does tend to reduce impact resistance but only the slowest reacting resin failed the impact test. Gloss readings were similar for all resins and comparable to the standard. Boiling water testing gives an indication of the cure conversion. Tests were performed after 6 and IO minutes curing at 200°C. The performance could not be connected to the amount of aliphatic end groups but appears to depend upon a high conversion rate. Exterior durability results showed little difference between the resins and it is assumed that at a constant resin composition the end groups do not influence the durability. In these trials a polyester/ crosslinker ratio of 96.513.5 was used. Increasing the amount of crosslinker is likely to improved overall performance. Paperuntitled “PolyesterResins with Reduced Reactivity for 6-hydroxyaikylamide Powder Coatings” by R Overkamp and coresearchers at DSM Coating Resins presented at PCE 2002 in Nuremberg on 15 17 Jan 2002. Bound copies of the conference papers are available from the organizers, Vincentz Veriag, Schiffgraben 43, D 30175 Hannover, Germany
INDUSTRY NEWS Diamond Vogel sets up powder coatings plant in Iowa Diamond Vogel has started the construction of a new powder 4
coatings manufacturing Orange City, IA. _.-
plant in
Tostem HomeWell starts using airpurifying paint
.~_
Modern Paint and Coatings, Jan-Feb 2002, 92 (I), 9
Nippon Paint takes position in plastic coatings Nippon Paint has entered the market for coatings for slimline notebook computer casings and mobile telephones. It is using its know-how in powder coatings and zero-solvent coatings for the ITrelated trim coatings market. It is hoping to achieve a turnover of Yen 3 bn in 3 years from this business. JapanChemicalWeek, 7 Feb 2002, 43 (2158), 3
Dow’s way to ultraviolet curing technology The ultraviolet (UV) curing industry is growing at 10%/y, and in 2002 a turnover of $2.3 bn is expected. Following the merger with Union Carbide, Dow now has Cyracure cycloaliphatic epoxide resins for UV technology. New products include epoxy acrylates, Vorastar urethane acrylates and UV resins for powder coating. .._.~ “~__ Around Dow, JaniFeb 2002, 24 (388), 8 (Dow Benelux NV, Terneuzen, The Netherlands. Tel: t 0115 671234) (in Dutch)
E Merck launches new generation pigments in Indian market E Merck India Ltd has introduced new generation pearl luster pigments in India, with wide range of industrial applications in plastics, textiles, printing inks, paper and automotive segments. The Xirallic and ColorStream ranges have been simultaneously introduced in India and abroad. The company’s lriodine Pearl Luster Pigments are available as free flowing dry powder with applications in screen printing, flexo, gravure, and offset printing inks. The Xirallic range provides effective pigments for coatings and other applications. ChemicalWeekly, 19 Feb 2002, 47 (26), 102
Tostem ~ome~ell Co has teamed up with paint producer Ahtech to develop a paint made from charcoal powder mixed with a resin containing carbon and nitrogen. The paint is claimed to purify the air inside buildings, and contains no volatile organic compounds. Sales of Yen 600 M are targeted for the first year. NikkeiNet, 26 Feb2002 (Website: h~p://www.nni.nikkei.co.jp/)
Akzo tdobel announced expansion of powder coatings activities in Italy Akzo Nobel Coatings is investing EUR 12 million in its powder coatings facility at Cernobbio in Italy. The move represents the launch of an expansion plan which will double powder production capacity at the site. “Italy is the largest powder coatings producing country in Europe and the second largest in the world after the United States,” said Rudy van der Meer, member of the Akzo Nobel Board of Management responsible for Coatings. “Powder coatings were introduced in Italy in the 1960s and are now firmly established as the number one finishing choice across a wide range of market sectors. When complete, this overall expansion plan will make our Cernobbio plant the largest single production unit for thermosetting powder coatings in the world. This investment is a significant step and will further strengthen our position as global market leader in powder coatings.” ~ ._ .~... ~~_ European Coatings Net. Websi~e: bttp://www.coatings.de (1I Mar 2002)
Akzo
Nobel plans 1500 more job cuts
Akzo Nobel is to axe a further 1500 jobs in its chemicals and coatings businesses because both posted lower profits in 2001. The cuts are on top of 2000 last summer. The chemicals segment APRlL
2002
Continuous powder coating spray process The standard method for manufacture of thermosetting powder coatings has dominated the market for almost forty years and a number of alternative methods have been developed in recent years although these alternatives have yet to become commercially acceptable. The main drawbacks to the traditional manufacturing technique are: high risk of cross contamination; broad particle size distributions; it is not suitable for low melting point coating systems; and is also unsuitable for processing highly reactive systems. Some years ago, the novel
VAMP process was developed by researchers at Ferro. This process replaced the extrusion, cooling and milling stages of manufacture with a stirred autoclave and a spray tower. In the autoclave a mixture of powder coating components was made with super critical carbon dioxide and, after processing, was sprayed to give a uniformly powdered material. Unfortunately, the costs of this process were prohibitive, due to the large high pressure vessel required for homogenization, the relatively long time in the autoclave, and the discontinuous mode of operation. Another continuous process for manufacture has now been devised based on the PGSS (Particles from Gas Saturated Solutions) technology. In this CPCSP (Continuous Powder Coating Spray Process), the main components of the powder coating mixture, the binder and the hardener are melted in separate vessels to avoid premature reaction of the polymers. Using high pressure pumps, the melts are dosed to a static mixer, and additionally carbon dioxide is dosed to the static mixer with a membrane pump. In the mixer the streams are homogenized and under pressures from 60 to 250 bar the gas is partially dissolved in the melts. This partial solution is then instantly released into a spray tower through a nozzle. The rapid expansion of the gas creates a Joule-Thomson effect which freezes the melt into fine droplets. The new process has been tested with single powder coating components and with complete formulations in polyester, epoxy and acrylic systems. In these experiments, the effect on particle size and distribution by varying the temperature, pressure and type of nozzle was examined. It was demonstrated that tailormade spherical particles with particle size lower than 40
microns and a very narrow particle size distribution could be obtained. Magnified photographs are shown in this paper to illustrate the sphericity of two polyester grades and a rough particle shape for an acrylic powder coating that is claimed to be caused by low process temperatures, resulting in high melt viscosities during the spray process. Also included are graphical representations of the effect of increasing pressure on particle size and the particle size distribution, and the phase behaviour of a polyester system in the presence of carbon dioxide. When the pressure is set at 61 bar only 7wt.% of carbon dioxide is dissolved in the melt and the powder formed with this setting is very coarse and shows a wide particle size distribution. Increasing the pressure to 177 bar causes nearly 16wt.% of carbon dioxide to dissolve in the melt and smaller particles with narrow particle size distributions are formed. Sensitivity of the process is shown by increasing the pressure to 190 bar when only lwt.% extra of carbon dioxide is dissolved in the melt, the particle size is further reduced and an excellent particle size distribution is obtained. Work on this project has been carried out with a laboratory sized plant capable of producing 15 kg of powder per hour. This has already been scaled up to a 50 kg/hour pilot plant and it is hoped to increase capacity to 350 kg/hour. It is claimed that there is a relatively small gas consumption of 0.5-2.0 kg of carbon dioxide per kg of powder coating. Paper entitled“Continuous PowderCoating SprayProcess - A New Technique to
Manufacture Powder Coatings” by M Petermann, E Weidner and V Rekowski of Ruhr University Bochum, and DuPont Performance Coatings, Germany, presented at PCE 2002 conference in Nuremberg on 15-17 Jan 2002. Bound copies of conference papers are available from the organizers, Vincent2 Verlag, Schiffgraben 43, 30175 Hannover, Germany
APRlL
2002
Reduced reactivity polyester resins for primid systems Over the past ten years, powder coatings based on polyesters crosslinked with a Primid hardener supplied by EMSChemie, have become well established. Their main uses are in outdoor applications for coating architectural facades, window profiles or industrial outlets in automotive primers, light fixtures and lawn and garden equipment. While the exterior durability and flexibility are features of these systems, the main criticisms are the high reactivity and the emission of volatile by-products during the curing cycle. The emission, which is usually water from the condensation reaction, combined with the entrapped air between the powder particles, results in problems with the degassing of the film. If the viscosity of the film is too high these trapped vapours will cause blistering and pinholing in the cured film. Improvements can be made by slowing down the viscosity rise and allowing most of these vapours to escape before the film is cured. The resins used for Primid cure are polyesters with carboxy functional end groups which react with the hydroxyl groups in the Primid to form ester bonds. This reaction does not proceed by the usual addition-elimination esterification reaction but via an oxazolinium-carboxylate ion pair intermediate mechanism which is much faster than normal esterification and cannot be controlled by catalysts. A paper by researchers at DSM Coating Resins describes the work that has been carried out to modify the reaction rate of the polyester component. Theoretically, there are several possible solutions to the degassing problem. A lower acid value in the polyester and a corresponding lower level of crosslinker would release water APRIL
2002
vapour at a lower rate, with a lower viscosity rise during cure. Unfortunately, this has an adverse effect on chemical resistance of the film and the corresponding higher molecular weight of the resin reduces the film flow. Degassing agents such as benzoin improve the release of air during cure but they can cause severe film yellowing. While pigments and additives do not affect the reactivity, the structure of the polyester can have a marked effect. A completely new approach is to modify polyester resins, maintaining composition and molecular weight, by altering the nature of the carboxylic end groups. It has been found that aliphatic end groups react slower than aromatic end groups, and that the end groups are responsible for the reactivity with the Primid crosslinker. Work at DSM has shown that reactivity can be influenced by the amount of aliphatic end groups, although side effects are a drop in glass transition temperature (Tg) and reduced water resistance. The use of adipic acid gives a drop in 7-g and a lower melt viscosity. The limitation to this approach is the storage stability of the finished powder coating. The usual components for powder coating polyester resins are: terephthalic acid, isophthalic acid and neopen~l glycol. DSM Resins has found that incorporating a constant level of aliphatic di-acids into the polyester backbone yields resins with different amounts of aliphatic acid end groups. The desired Tg of the polyester determines the amount of aliphatic acids that can be built into the backbone. It has been found that the reactivity of the resins can be varied independently of the Tg. This enables the properties like durability, powder stability and impact resistance to be retained while degassing qualities can be improved.
Physical and chemical properties of a range of experimental resins are tabulated and compared to the values for a comparable commercial resin. Adipic acid was used as the aliphatic acid component in these resins. Recorded acid values are in the same range as that of the commercial resin. The aliphatic acid expressed as a percentage of all end groups varied between 17.3% and 65.4% compared to 4.0% for the standard. Hydroxyl values are also recorded, together with Tg, melt viscosity, gel time, flow characteristics, and the film thickness limit before degassing is observed. All of these experimental resins showed lower reactivity and better degassing limits than the standard, whilst retaining all other properties. The resins with aliphatic end groups are synthesized in different ways. The different process conditions result in different amounts of adipic acid end groups and different degrees of conversion which is illustrated by the hydroxyl value. The reactivity of the crosslink reaction is measured by FT-IR analysis and by the gel time. Total conversion is independent of the amount of aliphatic end groups, but the conversion rate is related to the amount of aliphatic end groups. After IO minutes at the curing temperature, resins with a higher amount of aliphatic end groups have reached a lower end conversion since they have not completed their curing reaction, apart from the resin with the lowest amount of aliphatic end groups which has already reached its highest level of conversion, possibly due to the fast rate of reaction that restricts the mobility of the end groups and prevents further reaction. Gel time is also a measure of reactivity and this property depends on the amount of aliphatic end groups and the method of synthesis. Film flow
3
depends on rate of cure, but other factors can be viscosity, free acid content or particle size. The resins with aliphatic end groups flow as well as or better than the commercial standard. Degassing is found to improve with gel time and a lower reactivity increases the layer thickness at which blisters will occur. Impact resistance can only be achieved when a minimum amount of conversion is attained. In the tests with the trial resins, increasing the aliphatic end groups does tend to reduce impact resistance but only the slowest reacting resin failed the impact test. Gloss readings were similar for all resins and comparable to the standard. Boiling water testing gives an indication of the cure conversion. Tests were performed after 6 and IO minutes curing at 200°C. The performance could not be connected to the amount of aliphatic end groups but appears to depend upon a high conversion rate. Exterior durability results showed little difference between the resins and it is assumed that at a constant resin composition the end groups do not influence the durability. In these trials a polyester/ crosslinker ratio of 96.513.5 was used. Increasing the amount of crosslinker is likely to improved overall performance. Paperuntitled “PolyesterResins with Reduced Reactivity for 6-hydroxyaikylamide Powder Coatings” by R Overkamp and coresearchers at DSM Coating Resins presented at PCE 2002 in Nuremberg on 15 17 Jan 2002. Bound copies of the conference papers are available from the organizers, Vincentz Veriag, Schiffgraben 43, D 30175 Hannover, Germany
INDUSTRY NEWS Diamond Vogel sets up powder coatings plant in Iowa Diamond Vogel has started the construction of a new powder 4
coatings manufacturing Orange City, IA. _.-
plant in
Tostem HomeWell starts using airpurifying paint
.~_
Modern Paint and Coatings, Jan-Feb 2002, 92 (I), 9
Nippon Paint takes position in plastic coatings Nippon Paint has entered the market for coatings for slimline notebook computer casings and mobile telephones. It is using its know-how in powder coatings and zero-solvent coatings for the ITrelated trim coatings market. It is hoping to achieve a turnover of Yen 3 bn in 3 years from this business. JapanChemicalWeek, 7 Feb 2002, 43 (2158), 3
Dow’s way to ultraviolet curing technology The ultraviolet (UV) curing industry is growing at 10%/y, and in 2002 a turnover of $2.3 bn is expected. Following the merger with Union Carbide, Dow now has Cyracure cycloaliphatic epoxide resins for UV technology. New products include epoxy acrylates, Vorastar urethane acrylates and UV resins for powder coating. .._.~ “~__ Around Dow, JaniFeb 2002, 24 (388), 8 (Dow Benelux NV, Terneuzen, The Netherlands. Tel: t 0115 671234) (in Dutch)
E Merck launches new generation pigments in Indian market E Merck India Ltd has introduced new generation pearl luster pigments in India, with wide range of industrial applications in plastics, textiles, printing inks, paper and automotive segments. The Xirallic and ColorStream ranges have been simultaneously introduced in India and abroad. The company’s lriodine Pearl Luster Pigments are available as free flowing dry powder with applications in screen printing, flexo, gravure, and offset printing inks. The Xirallic range provides effective pigments for coatings and other applications. ChemicalWeekly, 19 Feb 2002, 47 (26), 102
Tostem ~ome~ell Co has teamed up with paint producer Ahtech to develop a paint made from charcoal powder mixed with a resin containing carbon and nitrogen. The paint is claimed to purify the air inside buildings, and contains no volatile organic compounds. Sales of Yen 600 M are targeted for the first year. NikkeiNet, 26 Feb2002 (Website: h~p://www.nni.nikkei.co.jp/)
Akzo tdobel announced expansion of powder coatings activities in Italy Akzo Nobel Coatings is investing EUR 12 million in its powder coatings facility at Cernobbio in Italy. The move represents the launch of an expansion plan which will double powder production capacity at the site. “Italy is the largest powder coatings producing country in Europe and the second largest in the world after the United States,” said Rudy van der Meer, member of the Akzo Nobel Board of Management responsible for Coatings. “Powder coatings were introduced in Italy in the 1960s and are now firmly established as the number one finishing choice across a wide range of market sectors. When complete, this overall expansion plan will make our Cernobbio plant the largest single production unit for thermosetting powder coatings in the world. This investment is a significant step and will further strengthen our position as global market leader in powder coatings.” ~ ._ .~... ~~_ European Coatings Net. Websi~e: bttp://www.coatings.de (1I Mar 2002)
Akzo
Nobel plans 1500 more job cuts
Akzo Nobel is to axe a further 1500 jobs in its chemicals and coatings businesses because both posted lower profits in 2001. The cuts are on top of 2000 last summer. The chemicals segment APRlL
2002